1. Field of the Invention
This invention pertains to a method and apparatus for accurately and simultaneously determining the volumetric flow rate of a fluid with a single fluidic oscillator without a priori knowledge of the physical properties of the fluid, and for determining the speed of sound in the fluid when the fluid properties are known.
2. Discussion of the Prior Art
Fluidic feedback oscillator flow meters and fluidic sonic oscillators are quite well known in the art (e.g., Horton et al, U.S. Pat. No. 3,185,166; Testerman et al, U.S. Pat. No. 3,273,377; Taplin, U.S. Pat. No. 3,373,600; Villarroel et al, U.S. Pat. No. 3,756,068; Adams, U.S. Pat. No. 3,640,133; Bauer, U.S. Pat. No. 4,244,230; and Zupanick, U.S. Pat. No. 4,150,561). These oscillators are all fundamentally similar devices comprised of a fluidic amplifier (digital or analog) with the outputs fed back on the inputs in such a manner as to produce free running oscillation. The period of oscillation fundamentally depends on three terms: the flow transit time of the fluid particles from the nozzle to the outputs; the feedback time through feedback channels, which is related to the acoustic transmission time; and a jet dynamic response time (see: H. Schaedel, xe2x80x9cFluidic Components and Networks,xe2x80x9d F. Vieweg and Son, Braunschweig/Weisbaden Germany, 1979). A fluidic flow meter is typically designed in such a manner that: (a) the flow transit time across the amplifier is made relatively long (i.e., by using a low velocity flow over a relatively long transit distance); (b) the acoustic feedback time is made relatively short by making the feedback length very short; and (c) the jet dynamic term is minimized by utilizing a high frequency response amplifier design. Accordingly, the flow transit time is very long compared with acoustic feedback time. In such a design the oscillator frequency is essentially linearly related to the volumetric flow because the flow transit time is related to flow velocity. Since the amplifier nozzle area is known, the product of velocity and area yields volumetric flow. In practice, the acoustic feedback time for most fluids can be designed to be only a few percent of the total flow transit time. Thus, for all intents and purposes, the measured frequency provides a relatively accurate measure of the flow rate, within the few percent represented by the acoustic time delay, regardless of the speed of sound in the fluid (e.g., for different fluids). By juxtaposition of time delays in a fluidic sonic oscillator that can be used as a speed of sound sensor, the acoustic feedback time is made to dominate by using physically long feedback lines; the flow transit time is made as short as possible by using a high velocity stream and short transit distance; and the jet dynamic term is again minimized by using a high frequency response amplifier design. In such a manner the oscillation period (and consequently the operating frequency) is essentially related, within the few percent attributable to the flow transit time, to the speed of sound in the fluid, provided that appropriate corrections are made to compensate for the retarding effects of the typically small dimensions of the feedback passages to account for differences in wall effects due to viscosity and density.
For certain applications, principally in assaying the constituents of a gas mixture (see my U.S. Pat. No. 6,076,392) it is necessary to measure both the flow of the fluid sample stream and the speed of sound in the fluid. A measure of the volumetric flow is required to determine the properties density and viscosity of the fluid sample, and a measure of the speed of sound is required to determine the property specific heat of thew fluid. In the latter case, the speed of sound sensor effectively becomes a gas calorimeter. In some embodiments described in my U.S. Pat. No. 6,076,392, two fluidic oscillators connected in series are employed, one oscillator providing mostly flow rate information and the other mostly speed of sound information. It is important to note that these two oscillator sensors are not completely decoupled because the degree of resolution and accuracy required for successful deconvolution of the composition equations is more than can be provided by each individual oscillator sensor alone. This is due to the abovementioned inherent errors arising from neglecting the small contributions of either flow or speed of sound. To correct for these inaccuracies, the frequencies from the two oscillators are used in an off-line simultaneous solution of sensor equations where, in essence, the speed of sound and flow dependence are solved for simultaneously with density and viscosity. The resultant numerical solution may be cumbersome and complex because of the number of roots that are often within a reasonable solution set. As a result it is often necessary to apply complex hierarchical decision trees to the numerical processing, and consequently the computational burden can become a limiting factor for practical implementation. Thus, when a low cost, lightweight system using a single microprocessor is desired, difficulties can arise. Further, when the oscillators are connected serially there is a time delay between when the first sensor reports an event (e.g., change in gas composition) and the second sensor experiences that same event, such time delay being a function of the transport delay. In order to prevent this out of phase information from affecting any computations, it is important to account for the time delay. Still further, the sonic oscillator, in general having a large volume associated with its feedback lines, requires more time to equilibrate (i.e., has a slower response) than the other sensing elements, further exacerbating any difficulties that may have been associated with the transient response. Additionally, the use of two oscillators often results in a phenomenon known as phase-locking whereby one oscillator may tend to lock into the frequency of the dominant oscillator, resulting in erroneous interpretation of that frequency. Even if phase-locking does not occur, there may often be some interaction which can affect the frequencies of both oscillators.
The technology of ultrasonic flow metering teaches that flow velocity can be determined by measuring the time delay between an acoustic signal injected upstream into a flow and an acoustic sensor disposed a known distance downstream. However, in its simplest form, this technique requires knowledge of the properties of the fluid since the time delay is actually the sum of the flow transit time superimposed on the sonic transit time. Using pairs of acoustic sources and detectors with one signal travelling upstream and the other downstream, subtraction of the inverse of the time delays cancels out the sonic terms, leaving only the flow terms which can be added to provide the acoustic propagation term. In any event, this type of device requires a separate acoustic source and is not particularly amenable to miniaturization for measuring the flow in small or even micro-channels.
The present invention resolves the abovementioned difficulties, particularly those of requiring two separate oscillators to determine volumetric flow and speed of sound, by recognizing that both flow and speed of sound are inherently part of the period of the frequency of a single oscillator, and that by using a separate means of measuring one of the component time delays, the other term can be automatically determined.
It is a primary object of the present invention to provide a method and apparatus utilizing a single fluidic oscillator with two phase-delayed measurements of the oscillating frequency to provide extremely accurate measures of the volumetric flow of the sampled gas and its speed of sound (and consequently its specific heat).
It is another object of the present invention to provide a volumetric flow meter and gas calorimeter using a single fluidic element.
It is a further object of the present invention to provide a method and apparatus utilizing two pairs of sensors to measure an actual time delay in feedback lines to correct the period of a flow meter frequency for changes in feedback delay caused by changes in propagation time due to changes in physical properties of a fluid with different or changing composition.
It is yet a further object of the present invention to utilize two pairs of sensors to measure an actual time delay of a flow stream to directly measure its flow rate, and subsequently to calculate the acoustic propagation velocity regardless of changes in physical properties of the fluid being monitored.
It is a further object of the present invention to provide an analytical method for determining the speed of sound and hence specific heat of a fluid when the fluid is a gas with a known relationship between speed of sound and specific heat.
Another object of the present invention is to provide a highly accurate volumetric flow meter for any fluid, gas or liquid, which is independent of the properties of the fluid being sampled, by direct compensation for the actual acoustic delay in the feedback lines.
Yet another object of the present invention to provide for a very low cost implementation of a flow meter and calorimetric device in order to promote widespread use and to improve the general standards of fluid flow and property measurement.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of the specific embodiments thereof, particularly when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components.